The invention described herein relates generally to joining. More specifically, the invention relates to a method of printing soldering and brazing preforms using additive manufacturing.
The stator windings in large generators may be water-cooled. The armature windings comprise an arrangement of half coils or stator bars (collectively referred to as “stator bars” or “bars”) connected at each end through copper or stainless steel fittings and water-cooled connections to form continuous hydraulic winding circuits. Water-cooled armature winding bars are comprised of a plurality of small rectangular solid and hollow copper strands arranged to form a bar. The rectangular copper strands are generally arranged in rectangular bundles. The hollow strands each have an internal duct for conducting coolant through the bar. The ends of the strands are each brazed to a respective hydraulic header clip. The hydraulic header clip serves as both an electrical and a cooling flow connection for the armature winding bar.
The hydraulic header clip is a hollow connector that includes an enclosed chamber for ingress or egress of a cooling liquid, typically deionized water. At one open end, the clip encloses the ends of the copper strands of the armature winding bar. A braze alloy bonds the end sections of the strands to each other and to the hydraulic header clip. The braze joints between adjacent strand ends and between the strand ends and the clip should retain hydraulic and electrical integrity for the expected lifetime of the winding. A typical life time of a winding is on the order of tens of years.
Internal surfaces of the brazed joints between the clip and the ends of the strands are constantly exposed to the deionized, oxygenated water flowing through the clip and the hollow strands. In addition, many other liquid filled conduits incorporate brazed joints exposed to water, such as phase leads, series loops, connection rings, bushings, as well as the many fittings needed to connect these conduits. The exposure of the brazed surfaces to the coolant/water can result in corrosion of conduits. Certain conditions promote crevice corrosion in the braze joints, such as: phosphorus, corrosive flux residues, copper, suitable corrosion initiation sites and water.
The corrosion process can initiate if the braze joint surfaces contain surface crevices, pinholes, or porosity at or near the surface of the joint and the critical water chemistry conditions that support corrosion. The corrosion process can progress through the braze joints especially when critical crevice geometry and water chemistry conditions exist. Porosity within the braze joints can accelerate corrosion. If allowed to progress through a joint, corrosion will eventually result in a water leak through the entire effective braze joint length and compromise the hydraulic integrity of the liquid filled conduits. Accordingly, there is a need for a corrosion-resistant brazed joint. The benefits of a corrosion-resistant brazed joint are expected to include improved generator availability and generator reliability.
Additive manufacturing processes, for example, may generally involve the buildup of one or more materials to make a net or near net shape object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), additive manufacturing encompasses various manufacturing and prototyping techniques known under a variety of names, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. Additive manufacturing techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer aided design (CAD) model. One exemplary additive manufacturing process uses an energy beam, for example, an electron beam or electromagnetic radiation such as a laser beam, to fuse (e.g., sinter or melt) a powder material, creating a solid three-dimensional object in which particles of the powder material are bonded together. Different material systems, for example, engineering plastics, thermoplastic elastomers, metals, and ceramics may be used. Laser sintering or melting is one exemplary additive manufacturing process for rapid fabrication of functional prototypes and tools.
Laser sintering can refer to producing three-dimensional (3D) objects by using a laser beam to sinter or melt a fine powder. Specifically, sintering can entail agglomerating particles of a powder at a temperature below the melting point of the powder material, whereas melting can entail fully melting particles of a powder to form a solid homogeneous mass. The physical processes associated with laser sintering or laser melting include heat transfer to a powder material and then either sintering or melting the powder material. Although the laser sintering and melting processes can be applied to a broad range of powder materials, the scientific and technical aspects of the production route, for example, sintering or melting rate, and the effects of processing parameters on the microstructural evolution during the layer manufacturing process can lead to a variety of production considerations. For example, this method of fabrication may be accompanied by multiple modes of heat, mass and momentum transfer, and chemical reactions.
Laser sintering/melting techniques can specifically entail projecting a laser beam onto a controlled amount of powder material (e.g., a powder metal material) on a substrate (e.g., build plate) so as to form a layer of fused particles or molten material thereon. By moving the laser beam relative to the substrate along a predetermined path, often referred to as a scan pattern, the layer can be defined in two dimensions on the substrate (e.g., the “x” and “y” directions), the height or thickness of the layer (e.g., the “z” direction) being determined in part by the laser beam and powder material parameters. Scan patterns can comprise parallel scan lines, also referred to as scan vectors or hatch lines, and the distance between two adjacent scan lines may be referred to as hatch spacing, which may be less than the diameter of the laser beam or melt pool so as to achieve sufficient overlap to ensure complete sintering or melting of the powder material. Repeating the movement of the laser along all or part of a scan pattern may facilitate further layers of material to be deposited and then sintered or melted, thereby fabricating a three-dimensional object.
For example, laser sintering and melting techniques can include using continuous wave (CW) lasers, such as Nd: YAG lasers operating at or about 1064 nm. Such embodiments may facilitate relatively high material deposition rates particularly suited for repair applications or where a subsequent machining operation is acceptable in order to achieve a finished object. Other laser sintering and melting techniques may alternatively or additionally be utilized such as, for example, pulsed lasers, different types of lasers, different power/wavelength parameters, different powder materials or various scan patterns to facilitate the production of one or more three-dimensional objects.